1. Field of the Invention
The present invention relates to a light signal generating device, and more particularly to a light signal generating device using an electro-optical material such as PLZT. The light signal generating device is used for image formation in a display, a printer, a facsimile or the like.
2. Description of Related Art
A light signal generating device using an electro-optical material, especially using PLZT which has a large Kerr constant has been known (refer to U.S. Pat. Nos. 4,902,111 and 4,932,761). The light signal generating device has a light shutter array comprising a line of light shutter elements, a polarizer and an analyzer which are disposed before and after the light shutter elements respectively. The light shutter elements are driven individually and selectively in accordance with image data. Light shutter elements which are impressed with a voltage transmits a light, and light shutter elements which are not impressed with a voltage shuts off the light.
Incidentally, it has been proved from an experiment that a light shutter element which is turned on and off frequently has a less light leakage (the quantity of light transmitted through a light shutter element in an off-state) than a light shutter element which is kept off. Practically, the light shutter elements are driven individually in accordance with the respective image data. Accordingly, the light shutter elements vary in the light leakage, and thereby, it is difficult to obtain a desired contrast.
For reproduction of a multitone image, the following method is adopted. A time required for reproducing a picture element is divided into some periods, and in connection with each light shutter element, it is decided in accordance with multitone image data in which period(s) the light shutter element is to be turned on (see Japanese Patent Laid Open Publication No. 1-272469). In this method, the number of times which each light shutter element is turned on depends on the desired tone level for the picture element, which is indicated by the multitone image data, and accordingly the light shutter elements vary in the driving frequency. Because of the different driving frequencies, drivers for impressing the respective light shutter elements with a voltage have different temperatures. In other words, the light shutter elements are exposed to different temperatures. A light shutter element made of PLZT or the like has a characteristic that its light transmittance varies in accordance with the circumstantial temperature. Therefore, the light shutter elements at different temperatures have different light transmittances, and it is difficult to reproduce fine multitone images stably.